It simply implies that while the scouts scoured Mirkwood for signs of the Black Riders, they stopped by Rhosgobel, likely with the intention of asking if he, or his animal friends, had seen anything.mas he wasn't there, and there were likely no signs of him nearby, the implication is that he is gone, or missing, from that area. But as he seems to be a well-travelled wizard, there is no indication that him not being at Rhosgobel, or in and around Mirkwood, constitutes 'missing.'

The primary objective of the scouts was to find information about the Black Riders, not to find Radagast. He was likely just one potential informant among many.